Coating dies are well known, and there are many variations. One typical coating die includes an upstream bar and a downstream bar connected together. An upstream die lip is part of the upstream bar, and a downstream die lip is part of the downstream bar. A manifold is formed in one or both bars and leads into a slot which exits the die at the lips of the die. Coating fluid is supplied through a channel to the manifold for distribution through the slot and coating onto a moving web or other surface to be coated. The coating fluid can form a continuous coating bead among the upstream die lip, the downstream die lip, and the surface being coated (such as a moving substrate or web). The coating fluid can be one of numerous liquids or other fluids. A vacuum chamber can apply vacuum upstream of the bead to stabilize the coating bead. The coating fluid can be applied to the web in a free span or against a backup roller.
A wide range of fluids are applied to surfaces using various coating dies. The coating dies themselves are modified for a specific application to optimize the coating of fluid on the surface. Varying other external features, such as the hardness of backup rolls, can also optimize coating. Within a given die configuration, the coating quality can be changed drastically by making small angle changes and height changes between the coating die parting line and a radial line extending outward from the center of the backup roll. Still, uneven coating persists often due to such factors as rubber roll runout or straightness, web line vibration, web thickness and surface variations, nonlinearity of the die lips, and the need for a precise placement of the die lip on the web. Coatings are sometimes inconsistent across their width due to an unevenness of the cross web die pressure.
The mounting systems for the coating dies are also modified to optimize coating. Coating dies are mounted at a location, sometimes called the coating station, at which the coating fluid is to be applied to a surface. Known mounting devices are rigid or fixed. That is, the die is positioned and fixed at a precise location adjacent the surface to be coated to optimize coating. This location can be changed to accommodate various fluids and coating conditions. However, during coating, the die remains stationary on its mount.
In some cases, coatings are applied to the web without any supporting member on the web backside at the point of coating application. An example of this is mayer rod coating where the web is supported between two rollers and a coating rod is mounted such that the web partially wraps around the rod circumference. The coating uniformity is controlled by, among other things, the rod straightness, web wrap angle around the rod, overall web tension, and the point-to-point web tension uniformity. If there is bag in the web at any point, that part of the web will have heavier coating than an adjacent area without bag that has a higher corresponding web tension.
In a similar manner, a pre-metered coating die can be pressed against a web in a free span between two rollers. Assuming that the coating die is designed properly for the coating liquid and provides a perfectly uniform crossweb distribution without the web in place, the final coating uniformity is a function of the localized web tension. A high web tension in one area of the die will cause the liquid to move towards an area having a lower localized web tension. Lower localized web tension in coating areas have higher coating thicknesses and higher tension areas have lower coating thicknesses. In extreme cases, the web tension will dominate the coating quality and cause skips and streaks.
U.S. Pat. No. 3,854,441 teaches using a press roll to apply force from the backside of the web against the die lips and the fluid exiting the die slot. Pneumatic cylinders provide pressure against the web and lift the press roll away from the web. However, very light pressures can not reliably be obtained because of friction in pneumatic cylinders sized to lift the weight of the roll.
GB Patent Publication No. 1190324 illustrates the use of a flexible blade to apply pressure between the web and the coating die. This is capable of very low pressures depending upon the flexibility of the blade. However, the blade rubs against and may scratch the web.
U.S. Pat. No. 3,609,810 discloses die coating where air lift bellows push and hold in position the die and its knife bar in close proximity to the web on a back-up roll. The gap between the die lip is fixed by pushing the die and the mounting fixtures against stops which may be adjusted to adjust the gap. With this arrangement, if the back-up roll is not perfectly round the gap will vary with each revolution. Urging the die against fixed stops does not allow self-compensation for out-of-roundness. Nor does it compensate for variation of the web thickness which will vary the gap.
Streaking can be more common when using contact extrusion die systems (as compared to non-contact systems) to apply thin coatings or fluids. In contact extrusion coating, the die position depends on whether fluid is exiting the die. If no fluid passes through the die, the die would contact with the substrate. When fluid passes through the die and exits through the die slot, the fluid flow causes a hydrodynamic pressure that moves the die away from the substrate, thereby opening a gap between the die lip and the substrate. This gap is known as the separation gap.
This contrasts with the metering gap which is a set, fixed clearance between a coater component (such as a knife edge) and the web. It is set by adjusting and fixing the mounting before coating. Thus, even when no fluid is being applied, the gap is still there. When fluid is applied, the gap is still there.
There is a need for a mounting system for a contact die coater in which the separation gap can be adjustable during coating to maximize coating quality.